tween the cornea and sclerotic is nine-twentieths, or nearly | irritation are suffered to produce their injurious effects half an inch in length. unheeded, and the eye soon becomes inflamed, ulcerates, and is destroyed.

the microscope to consist of hexagonal particles arranged of making the experiment, which show the appearance side by side like the cells of a honeycomb. It adheres more distinctly, but they are less simple. We shall offer an very loosely, so that when the surfaces covered with it are explanation of this experiment in treating on the physiodrawn to and fro in water, it becomes diffused, and may be logy of vision. The retina terminates anteriorly in a thin washed off. The choroid thus treated is found to be of scalloped edge, fitting into corresponding irregularities the same whitish or grey colour which characterizes the called the ora serrata in the posterior margin of the ciliary ciliary ligament. In the natural state of the parts, not only body. (Fig. 5, 7 and 9, d.) Exactly opposite the pupil there the choroid, but the cellular tissue on its external surface, is a bright yellow spot, fading gradually off at the edges, and and the inside of the sclerotic, are deeply stained by the having a black point in the centre precisely where the axis pigment, which shows through, and occasions the bluish of direct vision intersects the back of the eye. (Fig. 5, n.) tint of the white of the eye in persons of delicate com- This central point was believed by its discoverer, Soemplexion. But on the inner surface of the choroid the pig-mering, to be an actual deficiency of the substance of the ment is retained by an expansion finer than a spider's web, retina; and it is generally called in consequence the forayet of close texture, which may be called after its discoverer men of Soemmering. But it is now known to be merely a the membrane of Dalrymple. By this means the transpa- central absence of the yellow colour of that part of the reney of the retina is preserved. It is probable that this retina rendered conspicuous by the pigment seen through membrane of the pigment is of a serous kind, and that the ordinary transparent texture. These appearances are it is reflected at the optic and ciliary margins of the lost very shortly after death, and are replaced by a minute choroid, and passes over the whole posterior surface of fold, into which the retina gathers itself, reaching from the the retina, thus doubly defended from absolute contact place of the central point to the prominence which marks with the pigment. The choroid is not the only part which the union of the divided portions of the nerve. The use of secretes this coloring matter. It is found in equal or greater this yellow spot and central point, and of the tendency of abundance upon the back of the iris, on the surfaces of the the retina to assume a folded shape in this situation, is ciliary processes, and in fact wherever it is wanted to faci- not understood. It has been suggested that the group of litate vision. appearances is a rudiment of the yellow dye of the retina and of the pecten in birds, of which we shall give some account in a future part of the article. They are met with only in the eyes of man, the quadrumana, and some lizards. We have already described sufficiently the serous membrane which lines the posterior surface of the retina, supposed to be a reduplication of that which retains the pigment on the opposed surface of the choroid. Whatever may be the truth as to this supposition, there is no doubt of the existence of the retinal membrane, which was discovered by Dr. Jacob of Dublin, and has been named after him.

Tapetum Lucidum. In many animals, especially the nocturnal and carnivorous tribes, the pigment is deficient at the bottom of the eye, and the surface of the choroid in that situation presents a brilliant colour and almost metallic lustre. This is called the tapetum (tapestry of divers colours). It is of various shades of blue, green, and yellow; sometimes changeable like shot silk, and sometimes of a silvery whiteness. The tint occupies various proportions of the surface; it is most brilliant immediately opposite the pupil, and passes gradually into the dark hue of the pigment. There is no vestige of a tapetum in the human eye: the use of it is not well known. It probably causes the animal to see better in the dusk and less clearly in the day, by reflecting the rays a second time through the retina. This reflection from a very effective concave mirror produces a strong convergence of the rays which come back through the pupil, and is the cause of the well-known glare of the eyes of cats and other animals seen in an obscure light from that particular distance at which the emerging rays are most completely brought to a focus upon the eye of the observer. The breadth of this luminous appearance arises from the great dilatation of the pupil under the circumstances in which it is seen.

Retina. The optic nerve, having entered the interior of the globe through the sclerotic and choroid membranes, forms a slight prominence at the point of union of its several portions, and thence spreads out in the form of a fine transparent membrane over the whole concave surface of the choroid, embracing the translucent body called the vitreous humour. Towards the choroid it appears to consist of a mere homogeneous pulp not very different from the medullary matter of the brain; but it is undoubtedly most elaborately and minutely organized. Analogy would lead us to suppose it to have a fibrous arrangement, and recent observations of great nicety with the microscope appear to lead to the same conclusion.* Towards the vitreous humour it has the structure of a most delicate vascular web, consisting of innumerable ramifications of the central artery (which, as we have already mentioned, accompanies it into the interior of the globe) and of its associated veins. Its name may have been derived from the net-work formed by the visible branches of these vessels; at least it is not otherwise applicable to the structure of the membrane. The distribution of the central artery may be made visible to an observer in his own eye by a curious experiment first suggested by Purkinje. One eye being steadily directed to a surface of some uniform dark or neutral tint, such as a wall painted of a lead colour, and the other eye closed by the hand, the flame of a small wax taper is to be slowly waved round and round, so as to be brought at every turn at a little distance over the front of the eye. The central artery will gradually come into view, at first obscurely, and afterwards more clearly. The experiment succeeds best after the experience of several trials on successive nights. The form is such as might be expected from a branching net-work of vessels: the lines are dark, with bright edges on a faintly illuminated ground. There are other modes

• Müller's Physiology.

Vitreous Humour. (Fig. 5, e and 7, d.) The part next in order to be described is the vitreous humour, behind which the retina is disposed. It is a transparent fluid of semigelatinous consistence and high refractive power, constituting about five-sixths of the bulk of the globe. It consists of a fluid differing in no great degree from water, contained in a cellulated structure called the hyaloid membrane (valoc, glass), from its perfect translucency. The minute cells are connected together; for if the external part be punctured, the fluid contained in them gradually drains away. This cellular structure is so delicate and fragile that it is almost impossible to obtain it separately; but the membranous partitions are rendered slightly opaline by strong spirit or diluted acids, and may thus be

hyaloid membrane. The lens, imbedded in its upper surface, is surrounded This figure, from the work of Zinn, represents the vitreous humour in its by the canal of Petit. The dark border beyond is the plaited portion of the hyaloid membrane called the zone of Zinn, stained with the pigment left by tion of the surface of the hyaloid. The outer edge of the tone is marked by a the ciliary body, which in the natural state of the parts rests upon that porwaved outline, corresponding with the ora serrata of the ciliary body.

a, crystalline lens ; b, canal of Petit; c, zone of Zinn; d, vitreous humour. made evident. It is condensed into a membrane of a firmer consistence upon the surface, which serves the general purpose of a containing capsule for the vitreous humour, and is strong enough to cause it to preserve its shape in some degree when the stronger tunics of the eye are removed. There is a narrow tubular dimple of some depth in the vitreous humour opposite the entrance of the nerve, lined by a trumpet-shaped production into it of the external membrane, called the hyaloid canal. (Fig. 5.) It serves to transmit a branch of the retinal artery and associated veins for the nourishment of the capsule of the

Dalrymple,

lens in the foetus, and perhaps also of the hyaloid membrane, and of the substance of the lens itself. There can be no doubt that the vitreous humour is secreted by the surfaces of the hyaloid cells, but this foetal artery is the only vestige of a vascular arrangement yet discovered in the part.

Fig. 8.

pected to be obscure, considering that both the parts are diaphanous, and one of them almost liquid at the surface of contact. Indeed it has been supposed that a small quantity of limpid fluid was actually interposed between the lens and its capsule, but this is now shown to be the result of imbibition after death.

Besides its posterior attachment to the hyaloid membrane, the lens, or rather its capsule, has connections with other parts which adjoin it laterally in front: the nature of these will be best understood if those parts be first briefly described. At present, we shall only remark upon it further, that the gradual increase in its density from the circumference to the centre is a provision for correcting what is called the aberration of sphericity, or that impairment of focalising power which results from the too great refraction of the lateral rays of a pencil of light in passing through a homogeneous medium, such as glass, if bounded by spherical surfaces.

Several parts of much interest besides those we have already noticed are contained in the cavity of the aqueous humour, or form part of its boundaries. These are the iris, the ciliary body and processes, the zone of Zinn, and the canal of Petit.

Iris. (Fig. 5, h; 9, b.) In speaking of the choroid we have already adverted to the flattened ring called the ciliary ligament (Fig. 5, i) which connects it in front with the sclerotic. The iris arises from the anterior margin of this ring, and is extended, as we have seen, across the aqueous humour in the form of a thin partition with a round aperture, or pupil, of variable size in the centre, or a little nearer the inner side, the function of which, we need hardly repeat, is to regulate the quantity of light admitted into the eye, by contracting when it is in excess, and dilating when it falls short of the due amount.

Aqueous humour.-This fluid, in no respect distinguishable from 'vater except in holding a minute proportion of several salin. ingredients in solution, occupies the space between the lens and the cornea. The iris divides this space Lens, or Crystalline Humour (Figs. 5, f; 7, a; 8.) The into two unequal portions called the anterior and posterior crystalline (púoraloc, crystal) is imbedded in a deep de-chambers of the eye, and so closely approaches the lens that pression in the front of the vitreous humour, a little near the margin of the pupil the two surfaces are separated nearer the nasal than the temporal side of the globe. It by a mere film of aqueous humour. The cavity is lined has the form and function of a double convex lens. The throughout by a serous membrane which secretes and limits surfaces may be considered as portions of two unequal the fluid, and prevents it from acting injuriously upon subspheres, the anterior being considerably flatter than the jacent parts of importance. At least a membrane of this posterior. The diameter of the sphere of which the former kind may be peeled off in some animals; its existence in is a segment is about eight-twelfths, of the latter five- the human eye is rather a matter of inference than proof. twelfths of an inch. The thickness of the lens, measured in the axis of vision, is about the sixth part of an inch, and its transverse diameter is about twice that length. (Fig. 4, cc, d d.) In refractive power it is superior to the other transparent substances contained in the eye. Its consistence is gelatinous, increasing in density from the circumference towards a central nucleus, which has the tenacity of soft wax. It is composed of an infinite succession of thin concentric lamina, arranged with the utmost regularity one within another like the coats of an onion; and every such stratum or elliptic shell is made up of a series of exquisitely minute fibres laid side by side, forming three septa like the cloves of an orange, of which the bounding or cleavage planes diverge from the axis of the lens at angles of 120° in the manner represented in the annexed figure. (Fig. 8.) If the lens be hardened in strong spirit, the result of this curious arrangement is, that it partly cleaves into three portions made up of layers which may be peeled off one after another, each further separable to a certain extent into its component parallel fibres. The spirit not only hardens the crystalline humour, but renders it opaque; and the same effect is produced by plunging it in boiling water, as every one must have observed in the eyes of dressed fish. In fact it consists chiefly of the transparent substance called albumen found in eggs, and is coagulated by heat in the same way. The lens is similarly constructed in the eyes of other mammalia; and analogous, though not identical, arrangements are observed in other classes. In fish the fibres we have spoken of are curiously hooked together by fine teeth, resembling those of a saw. We chiefly owe the discovery of these facts respecting the intimate structure of the lens to the labours of Leeuwenhoek, Young, and Brewster, whose writings may be consulted for much further interesting information on the subject. The crystalline is enclosed in a transparent and highly elastic membranous capsule, represented in fig. 5 by a white line, to the regulated pressure of which the preservation of its true shape, so important to the purposes of vision, is mainly attributable. There is 1.0 analogous structure in the body, except the internal lining of the cornea already mentioned, which closely The external appearance of the iris is too familiar to need resembles it in its function with reference to the surface of a particular description. It is covered in front with a glisthe aqueous humour. When the lens is hardened in spirit tening polished membrane. The brilliancy of the eye or boiling water, this capsule retains its nature; and if depends in a great measure upon the light reflected by this peeled off, shrivels up and curls upon itself. It may be surface, and is lost when its smoothness and transparency easily detached with a pin from the hard spherical lens are impaired by inflammation. The posterior surface of the of a boiled fish, and will be found still possessed of its iris is called the uvea. (Fig. 9, b.) It is thickly coated peculiar qualities of transparency and elasticity. The cap- with pigment, which is prevented from diffusing itself in sule is firmly attached to the hyaloid membrane behind, the aqueous humour by a membrane like that of Dalrymple from which it is not easy to separate it neatly. Whether on the choroid. Such a provision is particularly needed it has any further connection than that of mere contact here on account of the quick movements of the part in a with its own contents, is not easy to show, or to doubt; but watery fluid. The colouring matter of the iris has much the nature of that connection, if any exists, might be ex-analogy with the pigment. Like that substance, it forms

Fig. 9.

and processes with the uvea as seen from behind when the lens is removed. Magnified view of a vertical section of the globe, showing the ciliary body a, Pupil; b, uvea, or back part of the iris; c, processes of the ciliary body; d, ora serrata of the ciliary body, to which a few shreds of the vascular web of the retina remain attached.

no part of the texture it pervades; and when the outer terminates, as we have seen, at the indented posterior marmembranes are removed by maceration in water, it may be gin (ora serrata) of this membranous band. The ciliary washed away. Both have a relation in quantity as well as body is everywhere thickly coated and pervaded with pigin depth of tint to the complexion and colour of the hair. ment, except at the extremities of about seventy minute In the negro the iris is of so dark a hue that it can scarcely unattached points which fringe the inner margin, and rabe distinguished from the pupil; while in the white rabbit diate towards the lens like the florets of a marigold round and other albinoes, including the human variety, where the its central disc. These are the ciliary processes. (Fig. 5, pigment is entirely wanting from some original malforma-k; 9, c.) They are separated from the uvea by the fluid of tion, the substance of the iris is transparent, and reflects the posterior chamber, and are received behind into correonly the pink colour of the circulating blood. Such eyes sponding depressions in the vitreous humour. are dazzled by a strong light, and probably see better than others in the dusk. The iris, if minutely injected, appears, like the choroid, to be composed almost entirely of vessels. It is principally supplied by the two long ciliary arteries (Fig. 6) which pierce the sclerotic about half an inch from the optic nerve on either side; and passing between that membrane and the choroid, divide near the edge and in the substance of the ciliary ligament, and are wholly distributed to the iris. Their branches are disposed in two conspicuous circles on the front surface, one near the outer or ciliary margin, the other not far from the pupil. But though the iris resembles the choroid in vascularity, it differs essentially from it in other respects. It is richly supplied with nerves, which proceed to the iris and are distributed upon it much in the same way as the arteries, and are the medium of its sympathy with the retina, and the source of its irritability. It also possesses a peculiar contractile power, thought by some to reside in fibres which they represent to be muscular, and to be disposed circularly in front and at the fore edge, and in a radiated form behind. (Fig. 9, b.) The former of these layers is supposed to contract and the latter to dilate the pupil. But this fibrous appearance may be deceptive, and is attributed by others to circular arrangements of the vessels and nerves, and to streaks and minute folds in the membrane itself.

Pupil. (Fig. 9, a.) The pupil in the human eye is bounded by a sharp well-defined circular edge. In other animals its shape is subject to many varieties which may often be explained by a reference to their habits and circumstances. In fish it is generally crescentic or imperfectly quadrangular. In herbivorous animals, which often continue to browse during the night, it is oblong and obliquely transverse, as in the horse and sheep. In most serpents and many rapacious quadrupeds, both aquatic and terrestrial, the pupil, though round and large at night, is a mere vertical slit when seen by day, especially in the smaller species of each genus, as in the common cat. It is curious that in the larger cats, as the lion and tiger, as well as in some of the larger four-footed reptiles, the pupil again becomes circular. In all birds, we believe, the pupil is round; and it may be observed that, with few exceptions, they all sleep after night-fall. In the few nocturnal species, as the owls, the pupil is very large though still round, and these birds always shun the day. The long narrow pupil i in fact a provision for a greater variation in size than the circular form permits, and is generally found in those animals which roam at night and also see well by day. When absent in such animals the bulk of the organ is commonly sufficient to secure the admission of a sufficient quantity of light after sunset without this provision. In the foetus the pupil is closed by a vascular film called the membrana pupillaris, one function of which is precisely that of the centering of a bridge, to support and extend it during the process of its construction. A tubular film of the same kind has been lately discovered by Müller stretched between the margin of the pupil and the ciliary body. Both these films are absorbed before birth.

Zone of Zinn.-If the ciliary body be carefully peeled off, and the thick radiated masses of pigment it leaves behind be washed away, a thin, transparent, and puckered membranous surface is exposed, extending from the indented margin of the retina nearly to the capsule of the lens, which appears to be something more than the mere external surface of the hyaloid membrane. This is called the zone of Zinn, from the anatomist who first drew attention to it. Various opinions are entertained of the nature of this zone. By some it is supposed to be a continuation of the vascular web, which may be called the internal tunic of the retina, arching round from the ora serrata, just as the tunic of Ruysch is continued into the ciliary body which lies upon the zone and corresponds with it in extent. The question is more curious than important in the present state of our knowledge of the functions of both of these delicate and elaborately constructed parts of the eye. The same remark may be extended to the controversies that subsist with respect to the part we have next to describe.

Canal of Petit (Fig. 7, b; 5, m). If the transparent membrane between the zone of Zinn and the margin of the lens be slightly punctured, and the point of a small blow-pipe be gently introduced, a canal may be inflated extending all round the lens in close proximity with the capsule, resembling a string of small glass beads laid in a circle. This is the canal of Petit. Whether the hyaloid membrane here separates into two layers, or whether the membrane of the aqueous humour lies upon it in this situation for a certain space without adhering, or how otherwise this canal is formed, it is not easy to say. In the natural state of parts it is empty and flaccid. When it is inflated the fine white triangular tips of the ciliary processes are seen to be received between its minute protuberances. But the points are loose and floating, and are not attached, as was formerly supposed, to the capsule of the lens.

Dr. Brewster has stated an opinion that the ciliary body is a muscular organ calculated to effect certain changes of curvature in the surfaces of the lens, or in its position, which seem to be required by the laws of refraction to account for the adjustment of the eye to different distances. Dr. Thomas Young was no less confident that the true solution of this optical enigma was to be found in the imagined muscularity of the fibrous structure of the lens itself. Both are high authorities, but neither opinion appears to have gained any ground.

Appendages of the Globe.-The eye-ball, of which we have thus described the contents, is lodged in the cavity of the orbit, a little nearer the inner than the outer side. In front, where the protection of bone is wanting, the two moveable and muscular eye-lids supply a sufficient defence, and contribute, by their gentle and constant pressure, to keep the eye in that state of equilibrium between opposite forces upon which the steadiness and precision of its rapid motions in a great measure depend. The space in the socket not occupied by the globe and its appendages is completely filled by a cushion of soft fat contained in elastic membranous cells, which permits the free movement of the Ciliary body and processes. (Fig. 5, 1; 9, dc). Upon the several parts, while it keeps them separate, and affords them compressed anterior surface of the vitreous humour where all, as well as the globe itself, a suitable and uniform supit curves inwards from the sclerotic towards the lens rests port. Varieties in the quantity of this substance, in the cathe ciliary body, a thin, dark, annular band, about the fifth pacity of the orbit, and in the development of the lids, depart of an inch in breadth, consisting of a frill of flat con- termine the different degrees of prominence and of apparent verging plaits, which encircle but do not reach the circum-size observed in the eyes of different persons; for the globe ference of the lens. The posterior aspect is concave, and itself is nearly of the same size in all. adheres loosely over the rounded vitreous humour; the front is convex, and is firmly attached to the whole breadth of the ciliary ligament, and to a small portion of the back of the iris near its junction with the ligament. It appears to be a continuation of the inner layer of the choroid, or tunic of Ruysch, but is rather thicker, and resembles it in extreme vascularity. The medullary matter of the retina One of the long ciliary arteries is represented in the figure by the middle white line; the narrower lines are the ciliary or iridian nerves,

Muscles of the Eye-ball.-The movements of the globe are effected by six muscles arising from the bony surface of the orbit, and inserted into different parts of the sclerotic. Four are called recti, that is straight or direct muscles; the fifth and sixth are the obliqui superior and inferior, so called from the obliquity of their insertion, and their respective positions above and below the globe. The fifth, or superior oblique, is also called the trochlearis, from the trochlea or pulley through which the tendon passes.

Lateral view of the right eye-ball, seen from the outer side, with its muscles. (This ent is to be considered only as a diagram, the proportios of the parts having been much altered to make them more distinct.),